S
Protoctista
Prokaryotes
The kingdom Protoctista (Figure 1.7) contains quite a mixture of organisms. They all have cells with a nucleus, but some have plant-like cells with chloroplasts and cellulose cell walls, while others have animal-like cells without these features. Most protoctists are unicellular (made of just a single cell) but some, such as seaweeds, are multicellular. Characteristics: ♦♦ multicellular or unicellular ♦♦ cells have a nucleus ♦♦ cells may or may not have a cell wall and chloroplasts ♦♦ some feed by photosynthesis and others feed on organic substances made by other organisms.
Figure 1.8 shows some bacteria. Bacteria have cells that are very different from the cells of all other kinds of organism. The most important difference is that they do not have a nucleus. You will meet bacteria at various stages in your biology course. Some of them are harmful to us and cause diseases such as tuberculosis (TB) and cholera. Many more, however, are helpful. You will find out about their useful roles in the carbon cycle and the nitrogen cycle, in biotechnology, in the treatment of sewage to make it safe to release into the environment and in making insulin for the treatment of people with diabetes. Some bacteria can carry out photosynthesis. The oldest fossils belong to this kingdom, so we think that they were the first kinds of organism to evolve. Characteristics: ♦♦ often unicellular (single-celled) ♦♦ have no nucleus ♦♦ have cell walls, not made of cellulose ♦♦ have no mitochondria.
Paramecium
cytoplasm
nucleus
cell membrane
S
Cross-section of the bacterium Escherichia coli flagellum strand of DNA plasmid
cell wall
Chlamydomonas
cytoplasm
cell wall
chloroplast
cell membrane
cytoplasm containing ribosomes
cell membrane
capsule
nucleus Figure 1.7 Some examples of protoctists.
External view of cholera bacteria Vibrio cholerae
Figure 1.8 Some examples of bacteria.
8
Cambridge IGCSE Biology
Original material © Cambridge University Press 2014